In the field of environmental hygiene air sampling is performed regularly to test the air in industrial work environments to determine the degree of exposure to hazardous chemicals. A typical sampling method involves collecting a sample of a test gas such as air by drawing a known volume of the test gas through a collecting device such as a sorbent tube. The sorbent tube may include a solid adsorbent capable of trapping and removing chemicals from the air or may include a filter for selectively collecting particulates. The test sample is then analyzed to determine the concentration level of the collected sample of chemicals or particulate matter. The method of analysis may involve gas chromatography or atomic adsorption, etc. The analysis is based on a time weighted average over an eight hour work day to determine the concentration level of contaminants in parts per million. For the analysis to be accurate it is essential that sampling of test gas be performed at a constant fluid flow rate which is preselected for the chemical hazard under examination.
Chemical sampling, i.e., sampling of gases or vapors, is typically conducted at low flow levels in a flow range typically between 1 to 250 cc per minute depending upon the chemical hazard under analysis. In the sorbent tube sampling method air is drawn through the sorbent tube by a vacuum pump at a flow rate which must be held constant to assure accurate results. One currently used technique to assure constant flow is to use a pump and counter in conjunction with a precalibrated known volume of test gas from which the total volume can be derived. This technique is nonetheless susceptible to erroneous results from changes in back pressure as a result of changes in pump volumetric efficiency, valve loading, etc. In another type of pump sampling system air flow is controlled by adjustment of the pump motor speed. One conventional system uses a pressure switch to generate output pulses which vary in duration corresponding to variations in flow rate. The pulsed output is electronically sensed and converted to control signals having an amplitude which varies with pulse duration. The control signals are then used to adjust the pump motor speed. Another known motor speed control system utilizes load sensing of the pump motor to adjust motor speed in proportion to the pump load line curve. The latter motor speed control system is shown and described in Applicant's earlier U.S. Pat. No. 4,432,248 issued February 1984. All known pump sampling systems which control flow by adjustment of pump motor speed produce an air flow with relatively high pulse undulations particularly at low flow levels. With a highly pulsed flow it is difficult to set the flow rate. In fact at very low flow rates of the order of 10 cc per minute or below a flow meter cannot be used to determine and calibrate the flow rate. Accordingly, at flow conditions it is still the practice to use a number of different pump systems to cover the low flow range with each calibrated to regulate flow in a very limited flow range.